TUNGSTEN OXIDE AND OXYGEN EVOLUTION REACTION CATALYST
An object of the present invention is to provide a compound with high catalytic activity that can be used as a catalyst for oxygen evolution reaction. Tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1). A catalyst for oxygen evolution reaction for use in an anodic electrode or a positive electrode, comprising the tungsten oxide. An electrolyzer comprising an anodic electrode chamber and a cathodic electrode chamber divided by an ion-permeable separating membrane, wherein an anodic electrode is arranged in the anodic electrode chamber and a cathodic electrode is arranged in the cathodic electrode chamber, and wherein the tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1) is supported on the anodic electrode as a catalyst.
The present invention relates to tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1), a catalyst for oxygen evolution reaction comprising the tungsten oxide, and a method for producing the tungsten oxide.
BACKGROUND ARTIn recent years, methods for producing hydrogen utilizing renewable energy have been attracting attention in order to solve problems such as global warming due to a greenhouse effect of carbon dioxide. The production of hydrogen utilizing renewable energy is required to lower cost so that it is comparable to that of the conventional method for producing hydrogen by reforming fossil fuels. Hydrogen production methods that can meet this requirement include electrolysis of water. A representative method of water electrolysis is alkaline water electrolysis. A power loss occurs upon the alkaline water electrolysis, and the main factors of the power loss include overpotential of an anodic electrode, overpotential of a cathodic electrode, an ohmic loss of an ion-permeable separating membrane, and an ohmic loss due to structural resistance of an electrolytic cell unit. If it is capable of reducing these power losses, the entire system can be miniaturized by enhancing a current density upon electrolysis in an electrolyzer, resulting in enabling significant reduction of equipment cost. Therefore, development of a catalyst capable of reducing the power losses has been desired.
Conventionally, ruthenium oxide, iridium oxide, and the like have been used as catalysts for oxygen evolution reaction, but these catalysts use precious metals, which are costly and limited resources. Therefore, utilization of tungsten oxide consisting of tungsten that is less costly and has more plentiful resources than precious metals, as a catalyst for the oxygen evolution reaction, has been investigated. Non-patent Document 1 describes Co1-xFexWO4-CNT, a composite of Co1-xFexWO4 and carbon nanotubes (CNTs), which is used as an oxygen evolution reaction (OER) catalyst. However, the document reports that the composite lowers its overpotential by being composited with carbon nanotubes, whereas overpotential of Co0.5Fe0.5WO4 without being composited with the carbon nanotubes is higher, with a value of 420 mV. Moreover, Non-patent Document 2 reports use of Ni—Fe—W hydroxide as a catalyst for OER, and it is also composited with carbon fibers. Therefore, there has been demand for a compound that exhibits high catalytic activity without using precious metals such as ruthenium and iridium.
PRIOR ART DOCUMENTS Non-Patent Documents
- Non-patent Document 1: Composite Metal Oxide-Carbon Nanotube Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions, ChemElectroChem, 5, 2850-2856(2018)
- Non-patent Document 2: Jie Xu, Mingshuo Wang, FeiYang, Xiaoqian Ju, Xilai Jia, “Self-Supported Porous Ni—Fe—W Hydroxide Nanosheets on Carbon Fiber: A Highly Efficient Electrode for Oxygen Evolution Reaction”, Inorg. Chem. 58, 13037-13048 (2019).
An object of the present invention is to provide a compound with high catalytic activity that can be used as a catalyst for oxygen evolution reaction.
Means to Solve the ObjectThe present inventors investigated a new compound with high catalytic activity that can be used as a catalyst for oxygen evolution reaction and have found that the compound represented by NixFe1-xWO4 (wherein 0<x<1) has very high catalytic activity. Although there was conventionally a literature describing use of various metal elements as M in the compound represented by MWO4, a compound in which M was a combination of Ni and Fe, by which very high catalytic activity could be obtained, was not known.
Namely, the present invention can be identified according to the items described below:
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- (1) Tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1).
- (2) A catalyst for oxygen evolution reaction for use in an anodic electrode or a positive electrode, comprising the tungsten oxide according to (1) above.
- (3) A method for producing tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1), wherein a tungstate salt, a nickel salt, and an iron salt are dissolved in a polyol, and a polyol solution in which the salts are dissolved is heated to synthesize the tungsten oxide, or wherein a tungstate salt, a nickel salt, an iron salt, and water are added into a pressure-resistant vessel and heated to synthesize the tungsten oxide.
- (4) An electrolyzer comprising an anodic electrode chamber and a cathodic electrode chamber divided by an ion-permeable separating membrane, wherein an anodic electrode is arranged in the anodic electrode chamber and a cathodic electrode is arranged in the cathodic electrode chamber, and wherein tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1) is supported on the anodic electrode as a catalyst.
- (5) The electrolyzer according to (4) above, comprising a gas diffusion layer for supplying carbon dioxide to the cathodic electrode, wherein the carbon dioxide is reduced in the cathodic electrode chamber.
- (6) The electrolyzer according to (4) above, comprising, on an opposite side of the cathodic electrode chamber, to a side facing the anodic electrode chamber, a carbon dioxide introduction part that introduces carbon dioxide so as to contact the cathodic electrode, wherein the carbon dioxide is reduced in the carbon dioxide introduction part.
- (7) A method for electrolyzing salt water, wherein, in the electrolyzer according to (4) above, salt water comprising alkali is supplied to the anodic electrode chamber and salt water is supplied to the cathodic electrode chamber to electrolyze the salt water.
- (8) A method for electrolyzing salt water as well as reducing carbon dioxide, wherein, in the electrolyzer according to (6) above, salt water comprising alkali is supplied to the anodic electrode chamber, salt water is supplied to the cathodic electrode chamber, and carbon dioxide is introduced into the carbon dioxide introduction part to electrolyze the salt water as well as reduce the carbon dioxide.
The tungsten oxide of the present invention exhibits excellent catalytic activity when used as a catalyst for oxygen evolution reaction. The catalyst for oxygen evolution reaction of the present invention exhibits excellent catalytic activity by comprising the tungsten oxide of the present invention. The production method of the present invention can produce the tungsten oxide of the present invention.
The tungsten oxide of the present invention is the compound represented by the chemical formula NixFe1-xWO4 (wherein 0<x<1). x is preferably 0.05 or more and 0.95 or less, preferably 0.10 or more and 0.90 or less, preferably 0.15 or more and 0.85 or less, preferably 0.2 or more and 0.8 or less, and more preferably 0.4 or more and 0.6 or less. Conventionally, ruthenium oxide, iridium oxide, and the like have been used as catalysts for oxygen evolution reaction, but the tungsten oxide of the present invention is superior in cost because it has a catalytic effect on oxygen evolution reaction without using precious metals such as ruthenium and iridium. Moreover, these metals have a problem of toxicity, whereas tungsten oxide has no toxicity problem. Furthermore, the tungsten oxide of the present invention is superior in catalytic activity than the case of using the precious metals. Conventionally, Co1-xFexWO4 (wherein 0<x<1), which is wolframite-type tungsten oxide comprising cobalt and iron, has been known as tungsten oxide with catalytic activity for oxygen evolution reaction, but the tungsten oxide represented by NizFe1-xWO4 (wherein 0<x<1), was not known. Known catalysts for oxygen evolution reaction consisting of nickel, iron, and tungsten are hydroxides and are different from the tungsten oxide of the present invention. The present invention is such that the tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1) was synthesized and found to be excellent as a catalyst for oxygen evolution reaction for one of the applications of the compound. The tungsten oxide of the present invention is preferably low-crystalline wolframite or a wolframite precursor that becomes wolframite upon heat treatment. Suitable crystallite sizes of the tungsten oxide of the present invention include 2.0 nm to 15.0 nm, 2.0 nm to 14.0 nm, 3.0 nm to 14.0 nm, or 3.0 nm to 7.0 nm.
The catalyst for oxygen evolution reaction of the present invention comprises the tungsten oxide of the present invention represented by NixFe1-xWO4 (wherein 0<x<1). The catalyst for oxygen evolution reaction of the present invention may consist only of the tungsten oxide of the present invention, or may comprise other compounds as long as it has catalytic activity, or may also be supported on a substrate such as a nickel foam, a carbon material, or a metal plate. The catalyst for oxygen evolution reaction of the present invention can be used for an anodic electrode or a positive electrode, for example, as a catalyst for oxygen evolution reaction in electrolysis, batteries, and the like, and for example, it can be used for an anodic electrode in water electrolysis, an air electrode (positive electrode) in metal-air batteries, and a counter electrode for reduction reaction in electrolysis of carbon dioxide. The tungsten oxide of the present invention used as a catalyst for oxygen evolution reaction can be a catalyst for oxygen evolution reaction with overpotential of 250 to 400 mV, 250 to 350 mV, or 300 to 350 mV to reach a current density of 10 mA/cm2. The tungsten oxide of the present invention can also be a catalyst for oxygen evolution reaction with a Tafel slope in the range of 30 to 50 mVdec−1, 30 to 45 mVdec−1, or 30 to 40 mVdec−1. In addition, the tungsten oxide of the present invention can be a catalyst for oxygen evolution reaction with an electrochemically active surface area in the range of 5 to 20 m2/g or 5 to 15 m2/g.
The method for producing the tungsten oxide of the present invention is not particularly limited, and examples thereof include a polyol method and a hydrothermal synthesis method. The polyol method is a method for dissolving a raw material salt in a polyol followed by heating the mixture to obtain a target product, and the hydrothermal synthesis method is a method for adding raw materials and water in a pressure-resistant sealed vessel and heating the mixture while sealing the vessel to obtain a target product. The polyol method comprises a step of dissolving various raw materials in a polyol and heating the polyol solution obtained in the above step, and when the tungsten oxide of the present invention is produced by the polyol method, the polyol used is not particularly limited, and examples thereof include ethylene glycol, propylene glycol, tetraethylene glycol, trimethylene glycol, tetramethylene glycol, diethylene glycol, dipropylene glycol, and polyethylene glycol. The salt to be dissolved in the polyol is salt comprising at least one of nickel, iron, and tungsten, which are the constituents of the tungsten oxide of the present invention, and is not particularly limited as long as it dissolves in the polyol for use, and these salts are used in combination and dissolved, so that the above three components are comprised in the polyol. Examples of tungsten sources include a tungstate salt. The tungstate salts include sodium tungstate, ammonium tungstate, and calcium tungstate. Nickel sources and iron sources include an acetate, a sulfate, a nitrate, and chloride of these sources. The heating temperature in the polyol method is not particularly limited, but is preferably a temperature in the vicinity of, or not higher than the boiling point of the polyol used as a solvent. The heating method is also not particularly limited, but is preferably refluxing at a temperature in the vicinity of the boiling point of the polyol used, since the most amount of heat can be applied at a normal pressure upon carrying out the synthesis reaction. A heating time can be appropriately selected as a time sufficient for the synthesis reaction to take place. For example, salt comprising nickel, salt comprising iron, and salt comprising tungsten are dissolved in a polyol. At this time, water may be added as appropriate, and the pH may be adjusted, if necessary. This solution is heated and refluxed. The heating temperature at this time varies depending on the type of polyol used, the amount of water added to the polyol, and the like, but it may be a temperature at which the aforementioned solution can be refluxed. The heating time is not particularly limited as long as the synthesis reaction is sufficiently carried out, and examples thereof include 30 minutes to 3 hours or 30 minutes to 2 hours. After heating, the temperature of the above solution is lowered to room temperature, and the solids can be collected by separation operations such as centrifugation to obtain the synthesized tungsten oxide of the present invention. Using a polyol as a solvent is considered to allow the polyol to act as a protective agent for a surface of the tungsten oxide particles produced and to prevent catalyst particles from growing due to aggregation thereof, resulting in enabling tungsten oxide having low overpotential, a large electrochemically active surface area, and high catalytic activity when used as a catalyst for oxygen evolution reaction, to be obtained. Moreover, the polyol method can synthesize tungsten oxide in a polyol at a normal pressure, thereby making it possible to be less expensive than a hydrothermal synthesis method requiring a pressure-resistant vessel.
In the case of producing the tungsten oxide of the present invention by the hydrothermal synthesis method, a raw material is not particularly limited as long as it is a substance dissolving in water at a predetermined temperature and pressure in a pressure-resistant vessel, and examples of tungsten sources include a tungstate salt. The tungstate salts include sodium tungstate, ammonium tungstate, and calcium tungstate. Examples of nickel sources and iron sources include respective salts thereof, which include an acetate, a sulfate, a nitrate, and a chloride. In the hydrothermal synthesis method, for example, a raw material for the tungsten source, a raw material for the nickel source, and a raw material for the iron source together with water are added into a pressure-resistant vessel and heated, and the mixture is reacted at a predetermined temperature and pressure for a predetermined time to enable production of tungsten oxide of the present invention. The temperature and pressure in the hydrothermal synthesis method can be appropriately selected according to the raw materials used, and examples of the temperature include 100 to 200° C., and the pressure in this case can be approximately 1 to 15 atmospheres. The synthesis time is not particularly limited as long as the synthesis reaction is sufficiently carried out, and examples of the synthesis time include 12 to 48 hours. Following the synthesis, the temperature and pressure in the pressure-resistant vessel are lowered, and the solids can be collected to obtain the tungsten oxide of the present invention.
The electrolyzer of the present invention comprises an anodic electrode chamber and a cathodic electrode chamber divided by an ion-permeable separating membrane, wherein an anodic electrode is arranged in the anodic electrode chamber and a cathodic electrode is arranged in the cathodic electrode chamber, and wherein the tungsten oxide of the present invention represented by NixFe1-xWO4 (wherein 0<x<1) is supported on the anodic electrode as a catalyst. The ion-permeable separating membrane in the present invention is not particularly limited as long as it is an ion-permeable separating membrane that can be used in an electrolyzer for electrolysis of an aqueous solution and the like, and examples of the ion-permeable separating membrane include a porous membrane consisting of asbestos or modified asbestos, a porous separating membrane using a polysulfone-based polymer, a cloth using polyphenylene sulfide fibers, a fluorine-based porous membrane, a porous separating membrane such as a porous membrane using a hybrid material comprising both an inorganic material and an organic material, and an ion-exchange membrane such as a fluorine-based ion-exchange membrane. The ion-permeable separating membrane in the present invention preferably has low gas permeability, low electrical conductivity, and high strength. The anodic electrode in the present invention has the tungsten oxide of the present invention supported on a conductive substrate as a catalyst. The conductive substrate is not particularly limited as long as it is a substrate that can be used as an electrode for electrolysis, and examples of the conductive substrate include nickel, a nickel alloy, nickel iron, vanadium, molybdenum, copper, silver, manganese, platinum group elements, graphite, or chromium, or combinations thereof. The conductive substrate may be a rigid substrate or a flexible substrate. Examples of the rigid conductive substrate include an expanded metal and a punched metal, and examples of the flexible conductive substrate include a wire mesh woven (or knitted) with metal wire. The method for supporting the tungsten oxide of the present invention onto the conductive substrate and the amount loaded are not particularly limited as long as the tungsten oxide of the present invention can contact an electrolytic solution and act as a catalyst, and examples of the supporting method include a method for covering the entire or partial surface of the conductive substrate, or a method for adhering the tungsten oxide to the entire or partial surface of the conductive substrate. The cathodic electrode in the present invention is not particularly limited as long as it is a substrate that can be used as an electrode for electrolysis, but it usually comprises a conductive substrate and a catalytic layer supported on a surface of the substrate. The conductive substrate is not particularly limited as long as it is a substrate that can be used as an electrode for electrolysis, and examples of the conductive substrate include nickel, a nickel alloy, stainless steel, mild steel, or stainless steel or mild steel with nickel plating thereon. The conductive substrate may be, for example, a rigid substrate or a flexible substrate. Examples of the rigid conductive substrate include an expanded metal and a punched metal, and examples of the flexible conductive substrate include a wire mesh woven (or knitted) with metal wire. The catalytic layers of the cathodic electrode include a catalytic layer comprising of a precious metal or precious metal oxide, nickel, cobalt, molybdenum, or manganese, or oxides thereof. In the anodic electrode chamber and cathodic electrode chamber in the present invention, the aforementioned anodic electrode and cathodic electrode are arranged, respectively.
In the electrolyzer of the present invention, water comprising an electrolyte is supplied to the anodic electrode chamber and cathodic electrode chamber and electrolyzed, to produce oxygen in the anodic electrode chamber, and produce hydrogen in the cathodic electrode chamber.
Further, in addition to the anodic electrode chamber, separating membrane, and cathodic electrode chamber, another embodiment of the electrolyzer of the present invention may comprise a gas diffusion layer for supplying carbon dioxide to a cathodic electrode, and carbon dioxide may be reduced.
The present invention will be described by way of Examples, but the present invention is not limited to these specific embodiments.
Example 1In a beaker, 25 mL of diethylene glycol was added, and hydrochloric acid diluted with distilled water was added to adjust the solution to have a pH of 5.5. The temperature of the solution after pH adjustment was raised to 70° C., and thereto were added 0.63 g of nickel(II) acetate tetrahydrate and 0.48 g of iron(II) acetate, and the mixture was stirred strongly with a stirrer until it became uniform. The solution in the beaker was transferred to a four-necked flask, and a solution in which 1.67 g of sodium tungstate dihydrate was dissolved in 2.5 mL of distilled water was added, and the temperature of the mixture was raised to 220° C. in 15 to 20 minutes. This solution was refluxed at 220° C. for 1 hour under strong stirring. After the reflux, the solution was cooled naturally to room temperature. To the resulting mixed solution were added acetic acid and ethanol, and the mixture underwent centrifugation several times, then only distilled water was added, and the mixture underwent centrifugation several times. The residue was vacuum dried at room temperature for 5 hours to obtain tungsten oxide (p-Ni0.5Fe0.5WO4) incorporating nickel and iron at a ratio of 5:5.
Example 2Tungsten oxide (p-Ni0.2Fe0.8WO4) incorporating nickel and iron at a ratio of 2:8 was obtained by the same method as in Example 1, except that the amount of nickel(II) acetate tetrahydrate added was 0.25 g and the amount of iron(II) acetate added was 0.70 g.
Example 3Tungsten oxide (p-Ni0.9Fe0.2WO4) incorporating nickel and iron at a ratio of 8:2 was obtained by the same method as in Example 1, except that the amount of nickel(II) acetate tetrahydrate added was 1.0 g and the amount of iron(II) acetate added was 0.18 g.
Example 4In a beaker, 25 mL of distilled water was added, the temperature was raised to 70° C., and thereto were added 0.11 g of nickel(II) chloride hexahydrate and 0.09 g of iron(II) chloride tetrahydrate. This solution was adjusted to have a pH of 5.5 by a sodium hydroxide aqueous solution and stirred with a stirrer for 10 minutes. Then, to the stirred solution was added a solution in which 0.3 g of sodium tungsten(IV) acid dihydrate was dissolved in 10 mL of distilled water, and the mixture was stirred for another 10 minutes. The solution in the beaker was transferred to a Teflon® vessel and heated in an autoclave at 180° C. for 24 hours. After the heating, the solution was cooled naturally to room temperature. To the resulting mixed solution, ethanol was added, and the mixture underwent centrifugation several times, then only distilled water was added, and the mixture underwent centrifugation several times. The residue was vacuum dried at 60° C. for 12 hours to obtain tungsten oxide (h-Ni0.5Fe0.5WO4) incorporating nickel and iron at a ratio of 5:5.
Comparative Example 1In a beaker, 25 mL of diethylene glycol was added and hydrochloric acid diluted with distilled water was added to adjust the solution to have a pH of 5.5. The temperature of the solution after pH adjustment was raised to 70° C., thereto was added 0.97 g of iron(II) acetate, and the mixture was stirred strongly with a stirrer until it became uniform. The solution in the beaker was transferred to a four-necked flask, and a solution in which 1.67 g of sodium tungstate dihydrate was dissolved in 2.5 mL of distilled water was added, and the temperature of the mixture was raised to 220° C. in 15 to 20 minutes. This solution was refluxed at 220° C. for 1 hour under strong stirring. After the reflux, the solution was cooled naturally to room temperature. To the resulting mixed solution were added acetic acid and ethanol, and the mixture underwent centrifugation several times, then only distilled water was added, and the mixture underwent centrifugation several times. The residue was vacuum dried at room temperature for 5 hours to obtain wolframite-type tungsten oxide (FeWO4) incorporating iron.
Comparative Example 2In a beaker, 25 mL of diethylene glycol was added and hydrochloric acid diluted with distilled water was added to adjust the solution to have a pH of 5.5. The temperature of the solution after pH adjustment was raised to 70° C., thereto was added 1.26 g of nickel(II) acetate tetrahydrate, and the mixture was stirred strongly with a stirrer until it became uniform. The solution in the beaker was transferred to a four-necked flask, and a solution in which 1.67 g of sodium tungstate dihydrate was dissolved in 2.5 mL of distilled water was added, and the temperature of the mixture was raised to 220° C. in 15 to 20 minutes. This solution was refluxed at 220° C. for 1 hour under strong stirring. After the reflux, the solution was cooled naturally to room temperature. To the resulting mixed solution were added acetic acid and ethanol, and the mixture underwent centrifugation several times, then only distilled water was added, and the mixture underwent centrifugation several times. The residue was vacuum dried at room temperature for 5 hours to obtain wolframite-type tungsten oxide (NiWO4) incorporating nickel.
Comparative Example 3WO3 (purity: 95.0%, Wako Pure Chemical Industries, Ltd.) was prepared and used as a sample for Comparative Example 3.
Comparative Example 4RuO2 (purity: 99.9%, Sigma-Aldrich Co. LLC) was used as a sample for Comparative Example 4.
Comparative Example 5IrO2 (purity: 99% (99.9+%-Ir), Strem Chemicals, Inc.) was used as a sample for Comparative Example 5.
Comparative Example 6In a beaker, 25 mL of diethylene glycol was added and hydrochloric acid diluted with distilled water was added to adjust the solution to have a pH of 5.5. The temperature of the solution after pH adjustment was raised to 70° C., thereto was added 0.63 g of nickel(II) acetate tetrahydrate and 0.48 g of iron(II) acetate, and the mixture was stirred strongly with a stirrer until it became uniform. The solution in the beaker was transferred to a four-necked flask, and the temperature was raised to 220° C. in 15 to 20 minutes. This solution was refluxed at 220° C. for 1 hour under strong stirring. After the reflux, the solution was cooled naturally to room temperature. To the resulting mixed solution were added acetic acid and ethanol, and the mixture underwent centrifugation several times, then only distilled water was added, and the mixture underwent centrifugation several times. The residue was vacuum dried at room temperature for 5 hours to obtain nickel iron oxide (p-NiFe oxide).
The samples obtained in Examples and Comparative Examples were characterized by the following methods.
(X-Ray Diffraction (XRD))XRD patterns were obtained by an X-ray diffractometer (Rigaku Ultima 4) equipped with CuKα radiation (40 kv, 40 mA).
(Linear Sweep Voltammetry (LSV))To a mixed solution comprising 350 μL of ethanol, 350 μL of water, and 95 μL of Nafion were added 5 mg of each sample and 5 mg of acetylene carbon black (conductive carbon), and the mixture underwent ultrasonic dispersion treatment for 60 minutes. 10 μL of the resulting dispersion was added dropwise to a disk electrode (5 mm in diameter) polished with alumina (amount of active material: 0.32 mg). The disk electrode was then dried at room temperature in air and used as a working electrode. A three-electrode cell was used, with a platinum mesh as a counter electrode and a Hg/HgO electrode (1M NaOH) as a reference electrode. 1M KOH purged with N2 for 30 minutes was used as an electrolytic solution. The sweep rate was set at 1 mV/s, and the rotation speed was set at 1600 rpm in order to remove oxygen bubbles on the working electrode. The resistance of a solution produced between the working electrode and the reference electrode was compensated for with a feedback rate of 60%. Oxygen evolution reaction produces protons, thereby lowering a pH of the electrolytic solution and changing water-oxidation potential. Calibration to a reversible hydrogen electrode (RHE) allows an effect of pH to be canceled. The equation ERHE=0.059×14+0.123+EHg/HgO was used for the calibration. The pH was 14.
(Cyclic Voltammetry)To a mixed solution comprising 350 μL of ethanol, 350 μL of water, and 95 μL of Nafion were added 5 mg of each sample and 5 mg of acetylene carbon black (conductive carbon), and the mixture underwent ultrasonic dispersion treatment for 60 minutes. 10 μL of the resulting dispersion was added dropwise to a disk electrode (5 mm in diameter) polished with alumina (amount of active material: 0.32 mg). The disk electrode was then dried at room temperature in air and used as a working electrode. A three-electrode cell was used, with a platinum mesh as a counter electrode and a Hg/HgO electrode as a reference electrode. 1M KOH purged with N2 for 30 minutes was used as an electrolytic solution. The sweep rate was set at 20 mV/s, and the electrode system was subjected to cycling (approximately 100 cycles) between 0 and +1 V, the range where the Faraday reaction was not observed.
(Durability Test (Chronopotentiometry; CP))To a mixed solution comprising 350 μL of ethanol, 350 μL of water, and 95 μL of Nafion were added 5 mg of each sample and 5 mg of acetylene carbon black (conductive carbon), and the mixture underwent ultrasonic dispersion treatment for 60 minutes. 10 μL of the resulting dispersion was added dropwise to a disk electrode (5 mm in diameter) polished with alumina (amount of active material: 0.32 mg). The disk electrode was then dried at room temperature in air and used as a working electrode. A three-electrode cell was used, with a platinum mesh as a counter electrode and Hg/HgO as a reference electrode. 1M KOH purged with N2 for 30 minutes was used as an electrolytic solution. The constant current density of 10 mA/cm2 was maintained for 24 hours. The working electrode was rotated at 1600 rpm as in linear sweep voltammetry.
The XRD patterns of the samples obtained in Example 1 and Comparative Example 2 are shown in
Tafel plots were created in order to analyze the rising portions in
Next, a reaction surface area (electrochemically active surface area: ECSA) of each of the samples obtained in Examples 1 to 4 and Comparative Examples 1, 2, and 6 was determined. For this purpose, an electrochemical double layer capacitance (Cdl), which is proportional to ECSA, was calculated for each sample by cyclic voltammetry (CV).
Turnover frequency (TOF) was determined for each of Examples 1 to 4 and Comparative Examples 1, 2, and 6 from the following equation. The results are shown in Table 3 and
TOF=[current density(Acm−2)×electrode surface area (cm2)]/[4×F (96485 Cmol−1)×total mole (mol) of transition metals of the catalyst on the electrode].
The XRD patterns, linear sweep voltammograms, and Tafel plots of p-Ni0.5Fe0.5WO4 fabricated in Example 1, the sample obtained by heating this at 600° C. for 3 hours, and p-NiFe oxide fabricated in Comparative Example 6 are shown in
Crystallite size=Kλ/(β cos θ)
(wherein K is a Bragg constant (=0.9), λ is a wavelength of X-ray used (CuKα radiation: 1.54051 Å), β is half width of the peak at 30°, and θ is a Bragg angle (½ of a diffraction angle 2θ)).
As a result, the crystallite size of p-Ni0.5Fe0.5WO4 without heating was 5.0 ran, the crystallite size of that heat-treated at 300° C. was 6.4 nm, the crystallite size of that heat-treated at 450° C. was 13.0 nm, and the crystallite size of that heat-treated at 600° C. was 16.8 nm. The tungsten oxide of the present invention is preferably low-crystalline wolframite or a wolframite precursor that becomes wolframite upon heat treatment.
Example 5To a mixed solution comprising 350 μL of ethanol, 350 μL of water, and 95 μL of Nafion were added 5 mg of the p-Ni0.5Fe0.5WO4 sample fabricated in Example 1 and −5 mg of acetylene carbon black (conductive carbon), and the mixture underwent ultrasonic dispersion treatment for 60 minutes. 10 μL of the resulting dispersion was added dropwise to a disk electrode (5 mm in diameter) polished with alumina (amount of active material: 0.32 mg). The disk electrode was then dried at room temperature in air and used as a working electrode. A three-electrode cell was used, with a platinum mesh as a counter electrode and a Hg/HgO electrode (1M NaOH) as a reference electrode. The electrolytic solutions used were a 0.5 M NaCl aqueous solution, an aqueous solution comprising 0.5 M NaCl and 0.1 M NaOH (+0.1 M NaOH), an aqueous solution comprising 0.5 M NaCl and 0.5 M NaOH (+0.5 M NaOH), an aqueous solution comprising 0.5 M NaCl and 1.0 M NaOH (+1.0 M NaOH), and an aqueous solution comprising 0.5 M NaCl and a 0.1 M borate buffer. 1 M KOH purged with N2 for 30 minutes was used. The sweep rate was set at 1 mV/s, and the rotation speed was set at 1600 rpm to remove oxygen bubbles on the working electrode. The resistance of the solution generated between the working electrode and the reference electrode was compensated for with a feedback rate of 60%. The oxygen evolution reaction produces protons, thereby reducing the pH of the electrolytic solution and changing water oxidation potential. Calibration to a reversible hydrogen electrode (RHE) allows an effect of pH to be canceled. The equation ERHE=0.059×14+0.123+EHg/HgO was used for the calibration. The obtained linear sweep voltammograms (LSV) is shown in
The same operations and treatments as in Example 5 were carried out except that the electrolytic solutions used were a 0.5 M NaClO4 aqueous solution, an aqueous solution comprising 0.5 M NaClO4 and 0.1 M NaOH (+0.1 M NaOH), an aqueous solution comprising 0.5 M NaClO4 and 0.5 M NaOH (+0.5 M NaOH), an aqueous solution comprising 0.5 M NaClO4 and 1.0 M NaOH (+1.0 M NaOH), and an aqueous solution comprising 0.5 M NaClO4 and a 0.1 M borate buffer in Example 5. The linear sweep voltammetry (LSV) obtained is shown in
In Examples 5 and 6, the Tafel slope and the overpotential (η) at 10 mA/cm2 were investigated. The pH of the 0.5 M NaCl aqueous solution in Example 5 was 5, the pH of the aqueous solution comprising 0.5 M NaCl and 0.1 M NaOH was 13, the pH of the aqueous solution comprising 0.5 M NaCl and 0.5 M NaOH was 13.5, the pH of the aqueous solution comprising 0.5 M NaCl and 1.0 M NaOH was 14, and the pH of the aqueous solution comprising 0.5 M NaCl and a 0.1 M borate buffer was 9. The pH of the 0.5 M NaClO4 aqueous solution in Example 6 was 5, the pH of the aqueous solution comprising 0.5 M NaClO4 and 0.1 M NaOH was 13, the pH of the aqueous solution comprising 0.5 M NaClO4 and 0.5 M NaOH was 13.5, the pH of the aqueous solution comprising 0.5 M NaClO4 and 1.0 M NaOH was 14, and the pH of the aqueous solution comprising 0.5 M NaClO4 and a 0.1 M borate buffer was 9. The Tafel slope at each pH and the overpotential (η) at 10 mA/cm2 is shown in
The same operations and treatments as in Example 5 were carried out except that IrO2 of Comparative Example 5 was used instead of p-Ni0.5Fe0.5WO4 in Example 5. The linear sweep voltammetry (LSV) obtained is shown in
The same operations and treatments as in Example 6 were carried out except that IrO2 of Comparative Example 5 was used instead of p-Ni0.5Fe0.5WO4 in Example 6. The linear sweep voltammetry (LSV) obtained is shown in
The 0.5 M NaCl aqueous solution in Example 5 is an aqueous solution that mimics seawater.
Furthermore, with the p-Ni0.5Fe0.5WO4 electrode used in Example 5, constant-current electrolysis was carried out at 10 mA/cm2 until the amount of electricity passed reached 40 C/cm2 (=10 mA/cm2×4000 s), and the residual chlorine concentration in the test solution after the electrolysis was measured by the general iodine titration method. The measurement was conducted by adding potassium iodide to the test solution after the electrolysis and titrating free iodine with a sodium thiosulfate solution. The Faraday efficiency of chlorine generation (chlorine generation efficiency) was determined by the following formula:
Faraday efficiency(%) of chlorine generation=([Cl2]×V)/{Q/(nF)}×100
(wherein n is the number of electrons involved (=2) (2Cl−→Cl2+2e−), V is volume of the test solution, Q is the amount of electricity passed, and F is a Faraday constant (=96,485 C/mol)).
The results showed that the Faraday efficiency of chlorine generation was 16% for the 0.5 M NaCl aqueous solution, 0.3% for the aqueous solution comprising 0.5 M NaCl and 0.1 M NaOH, 0.2% for the aqueous solution comprising 0.5 M NaCl and 0.5 M NaOH, 0.1% for the aqueous solution comprising 0.5 M NaCl and 1.0 M NaOH, and 0.3% for the aqueous solution comprising 0.5 M NaCl and a 0.1 M borate buffer, and these results also indicate that using the tungsten oxide of the present invention as a catalyst can inhibit generation of chlorine gas that is a toxic gas and can generate oxygen in electrolysis of salt water such as seawater. The IrO2 electrode used in Comparative Example 7 was also used to measure the residual chlorine concentration in the test solution after electrolysis by using the same test method and measurement method as for the p-Ni0.5Fe0.5WO4 electrode described above. It is found from the results that the chlorine generation efficiency was 47% for the 0.5 M NaCl aqueous solution, 0.6% for the aqueous solution comprising 0.5 M NaCl and 0.1 M NaOH, 0.5% for the aqueous solution comprising 0.5 M NaCl and 0.5 M NaOH, 0.1% for the aqueous solution comprising 0.5 M NaCl and 1.0 M NaOH, and 0.6% for the aqueous solution comprising 0.5 M NaCl and a 0.1 M borate buffer. These results indicate that the use of the tungsten oxide of the present invention as a catalyst can inhibit generation of chlorine species upon oxygen evolution more than the use of IrO2, which is conventionally used as a catalyst exhibiting high activity for the oxygen evolution reaction.
The tungsten oxide of the present invention can be suitably used as a catalyst for oxygen evolution reaction in electrolysis, batteries, and the like, and, for example, it can be used as an anodic electrode in water electrolysis, an air electrode (positive electrode) in metal-air batteries, and a counter electrode for reduction reaction in electrolysis of carbon dioxide. As for water electrolysis, the tungsten oxide of the present invention can be used as catalysts for electrolysis such as alkaline water electrolysis, alkaline seawater (salt water) electrolysis, alkaline CO2 electrolysis, and alkaline CO2 seawater (salt water) electrolysis. Further, since the electrolyzer of the present invention comprises the tungsten oxide of the invention as a catalyst, it can be suitably used as electrolyzers in various electrolysis such as water electrolysis, and for example, it can be suitably used as electrolyzers for electrolysis such as alkaline water electrolysis, alkaline seawater (salt water) electrolysis, alkaline CO2 electrolysis, and alkaline CO2 seawater (salt water) electrolysis. The production method of the present invention is suitable as a method for producing such tungsten oxides.
Claims
1. Tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1).
2. A catalyst for oxygen evolution reaction for use in an anodic electrode or a positive electrode, comprising the tungsten oxide according to claim 1.
3. A method for producing tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1),
- wherein a tungstate salt, a nickel salt, and an iron salt are dissolved in a polyol, and a polyol solution in which the salts are dissolved is heated to synthesize the tungsten oxide, or
- wherein a tungstate salt, a nickel salt, an iron salt, and water are added into a pressure-resistant vessel and heated to synthesize the tungsten oxide.
4. An electrolyzer comprising an anodic electrode chamber and a cathodic electrode chamber divided by an ion-permeable separating membrane, wherein an anodic electrode is arranged in the anodic electrode chamber and a cathodic electrode is arranged in the cathodic electrode chamber, and wherein tungsten oxide represented by NixFe1-xWO4 (wherein 0<x<1) is supported on the anodic electrode as a catalyst.
5. The electrolyzer according to claim 4, comprising a gas diffusion layer for supplying carbon dioxide to the cathodic electrode, wherein the carbon dioxide is reduced in the cathodic electrode chamber.
6. The electrolyzer according to claim 4, comprising, on an opposite side of the cathodic electrode chamber, to a side facing the anodic electrode chamber, a carbon dioxide introduction part that introduces carbon dioxide so as to contact the cathodic electrode, wherein the carbon dioxide is reduced in the carbon dioxide introduction part.
7. A method for electrolyzing salt water, wherein, in the electrolyzer according to claim 4, salt water comprising alkali is supplied to the anodic electrode chamber and salt water is supplied to the cathodic electrode chamber to electrolyze the salt water.
8. A method for electrolyzing salt water as well as reducing carbon dioxide, wherein, in the electrolyzer according to claim 6, salt water comprising alkali is supplied to the anodic electrode chamber, salt water is supplied to the cathodic electrode chamber, and carbon dioxide is introduced into the carbon dioxide introduction part to electrolyze the salt water as well as reduce the carbon dioxide.
Type: Application
Filed: Oct 8, 2021
Publication Date: Nov 23, 2023
Inventors: Masaharu Nakayama (Yamaguchi), Airi Takeda (Yamaguchi), Heishi Maruyama (Yamaguchi), Masanobu Azuma (Yamaguchi)
Application Number: 18/030,904